The invention relates to a method of manufacturing a polycrystalline semiconductor resistance layer of silicon on a silicon body, in which first an insulating layer is formed on the silicon body and then a polycrystalline silicon layer is deposited, which layer is then doped. The invention further relates to a silicon pressure sensor having such a resistance layer.
Such a method is known from the article "Silicium-Drucksensor fur hohe Betriebstemperaturen" by E. Obermeier and F. von Kienlin, Proc. Sensor 85, Transducer-Technik: "Entwicklung und Anwendung", Karlsruhe, 1985, pp.4.3.1 to 4.3.12. In this case, a structural resistance layer of polycrystalline semiconductor silicon is applied to a pressure sensor, which consists of a silicon body and which is arranged on a carrier. The silicon body has a cavity in the form of a blind hole, as a result of which a diaphragm is formed. The structured resistance layer is applied to the outer surface of the diaphragm.
During the manufacture of the resistance layer, an oxide layer (silicon dioxide) is formed as an insulating layer on the diaphragm by thermal oxidation. Subsequently, a deposition of a polycrystalline silicon layer by Low Pressure Chemical Vapour Deposition (LPCVD) takes place with silane as reaction gas in a conventional diffusion oven. The layer is then doped by ion implantation and the resistance layer obtained is structured after a thermal curing process by means of a photoetching process. The resistance layer is contacted by vapour deposition and structuring of an aluminum layer. The crystallites obtained by thermal curing in the polycrystalline layer have an average grain diameter of 80 nm to 200 nm.
The measure for the sensitivity of expansion of a resistor R is its k factor, which indicates the relative resistance variation dR/R per expansion e: EQU dR/R=ke.
The resistance layer manufactured by means of the method described above has a k factor between 27 and 35.
Instead of a curing process after deposition of the polycrystalline layer, as proposed in the aforementioned publication, after deposition a laser recrystallization may also be used for forming enlarged crystallites (grains), as is known from "Laser-Recrystallized Polysilicon Resistors for Sensing and Integrated Circuits Applications" by J. Binder, W. Henning, E. Obermeier, H. Schaber and D. Cutter, Sensors and Actuators, 4, 1983, pp. 527 to 536. In this case, the deposited polycrystalline silicon layer is fused for a short time in the focus of a laser. During the solidification of the fused silicon, enlarged crystallites are formed having an average grain diameter between 1000 nm and 10,000 nm. The k factor for the polycrystalline layer manufactured by means of the laser-recrystallization lies between 45 and 55. It is found therefore, that the sensitivity to expansion of the resistance layer depends upon the size of the crystallites. The laser recrystallization is very complicated, however, because the whole surface of the polycrystalline silicon layer must be fused pointwise. In order that the light energy of the laser can be coupled into the silicon layer, the polycrystalline silicon must moreover be covered by an additional transparent nitride layer, this nitride layer provides mechanical stabilization of the liquid polycrystalline layer during the laser scanning and an antireflex coating, in order that the light is not reflected by the silicon layer.